Add component database layer and validated physics core

- components/schema.py + database.py: typed loader for real retail parts
  (wire, capacitors, switches, sensors, projectile materials)
- physics/inductance.py: multilayer solenoid inductance (Wheeler) with
  ferromagnetic-slug coupling via smooth overlap model and analytic dL/dx
- physics/force.py: F = 0.5*I^2*dL/dx with a saturation clamp
- physics/circuit.py: coupled [Q,I,x,v] discharge ODE
- Validated against textbook RLC analytical solutions (under/over/critically
  damped) and energy conservation, not just spot-checked by eye
- PLAN.md updated with animation (per-run field visualization) and
  GPU-accelerated batch sweep as explicit later stages

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_force_model.py

@@ -0,0 +1,35 @@
+import pytest
+
+from gausse.physics.force import (
+    force_on_slug_newtons,
+    saturation_scale,
+    solenoid_field_estimate_tesla,
+)
+
+
+def test_field_estimate_scales_with_current_and_turns():
+    b_low = solenoid_field_estimate_tesla(mu_eff=100, total_turns=100, coil_length_m=0.05, current_a=10)
+    b_high = solenoid_field_estimate_tesla(mu_eff=100, total_turns=100, coil_length_m=0.05, current_a=100)
+    assert b_high > b_low
+
+
+def test_saturation_scale_is_one_below_bsat():
+    assert saturation_scale(b_estimate_tesla=0.5, b_sat_tesla=1.8) == pytest.approx(1.0)
+
+
+def test_saturation_scale_clamps_above_bsat():
+    scale = saturation_scale(b_estimate_tesla=3.6, b_sat_tesla=1.8)
+    assert scale == pytest.approx(0.5)
+
+
+def test_force_is_reduced_once_saturated():
+    common = dict(dl_dx_unsaturated=1e-3, mu_eff=200, total_turns=300, coil_length_m=0.05, b_sat_tesla=1.8)
+    force_unsaturated = force_on_slug_newtons(current_a=5, **common)
+    force_saturated = force_on_slug_newtons(current_a=500, **common)
+    # без клэмпа сила росла бы как I^2 (в 10000 раз); с насыщением рост должен быть намного меньше
+    assert force_saturated / force_unsaturated < 5000
+
+
+def test_force_zero_at_zero_current():
+    common = dict(dl_dx_unsaturated=1e-3, mu_eff=200, total_turns=300, coil_length_m=0.05, b_sat_tesla=1.8)
+    assert force_on_slug_newtons(current_a=0, **common) == pytest.approx(0.0)